1. Field of the Invention
This invention relates to heaters for construction sites and more particularly to providing a gas-fired construction heater that complies with nationally-recognized standards.
2. Discussion of Prior Art
In the past, a major telephone company approved certain types of gas-fired construction heaters based on its standards (the "Bell Standards"). Heaters complying with such standards are referred to in the manual "Outside Plant Construction--Safety, Tools, General;" No. 200-326, Issue 2, December 1979, published by Western Electric Company, Incorporated for American Telephone and Telegraph Company. Such heaters were used by many telephone operating companies in the maintenance of telephone equipment. Such operating companies would only purchase heaters that had such approval. Therefore, manufacturers sought such approval for their gas-fired construction heaters.
There have been major changes in the organization of the major telephone company in the United States of America. One result of these changes is that no single telephone company continues to set a nationally-recognized standard for these types of gas-fired construction heaters. Consequently, manufacturers of such heaters have sought a different way of assuring their customers that such types of heaters are suitable for the customers' purposes, such as construction and use in maintaining telephone equipment. For example, for one type of gas-fired construction heater sold in the United States of America, one possible way of providing such assurance is to design the gas-fired construction heater so as to comply with applicable standards of the American National Standards Institute ("ANSI"). The current ANSI Standard for gas-fired construction heaters is identified as ANSI Z83.7b-1989 (the "ANSI Standard"). The ANSI Standard is used by equipment certifying organizations, such as the American Gas Association and the Underwriters Laboratories. These organizations test equipment, such as gas-fired construction heaters, and certify the equipment if the equipment meets the ANSI Standard. As to gas-fired construction heaters to be sold in Canada, the Canadian Gas Association ("CGA") has established many standards, which currently are different from the ANSI Standard.
However, the ANSI Standard is in a number of respects more stringent than both the Bell Standards and the current CGA standards. For example, the ANSI Standard will not permit a gas-fired construction heater to have a manually adjustable primary combustion air inlet. As a result, personnel using a gas-fired construction heater conforming to the ANSI Standards in the field will not be able to adjust primary combustion air flow in the heater according to actual field conditions.
Further, the ANSI Standard requires such a gas-fired construction heater to either operate acceptably or shut itself off, and the acceptable operation must occur despite variations in uncontrolled operating conditions within predetermined ranges. For example, gas-fired construction heaters are designed using a standard heat value of 2500 BTU per cubic foot of propane burned. A gas-fired construction heater that is so designed and "rated" for 45,000 BTU would burn 18 cubic feet of propane per hour. That amount of propane would be supplied by selecting (1) the size of the burner orifices and (2) the standard gas pressure in the manifold ("manifold pressure") that supplies gas for the burner, with the selections taking into consideration the barometric pressure under which the burner is to operate. The term "input rate" as used herein indicates fuel supply to the burner at a standard manifold pressure for a particular size burner orifice, a known heat value of the fuel, operation of the burner at a specified barometric pressure and for a given temperature of fuel admitted to the burner. The "normal input rate" is the input rate of such heater as specified by the manufacturer of such heater. The ANSI Standard requires gas-fired construction heaters to operate in a range of from 85% to 112% of the normal input rate without producing more than 0.08% carbon monoxide ("CO") air-free in a random sample of the combustion products of the gas-fired construction heaters (the "CO Standard"). Thus, the range in which the input rate of the gas-fired construction heaters must operate while meeting the CO Standard is said to be "predetermined" and varies from 85% to 112% of the normal input rate. The term "reduced rate" as used herein indicates fuel supply at 85% of the normal input rate, and the term "increased rate" indicates fuel supply at 112% of the normal input rate.
Air supplied to the gas-fired construction heaters is another uncontrolled operating condition to which the more stringent ANSI Standard relates. Gas-fired construction heaters may have a blower that supplies fresh air to a heat exchanger which heats the air. The blower may also supply combustion air to the burner. Since gas-fired construction heaters are used at construction sites at which portable electric generators are used, the blowers are generally driven by an electric motor. Because such portable electric generators tend to output variable voltage, the ANSI Standard requires gas-fired construction heaters to operate in a range of from 85% to 110% of a standard supply voltage without exceeding the CO Standard. Thus, the gas-fired construction heaters must operate and meet the CO Standard in response to a range of such voltage that is said to be "predetermined" and varies from 102 VAC to 132 VAC if a standard supply voltage of 120 VAC is used. The term "reduced voltage" as used herein indicates that 85% of the standard supply voltage is supplied to the heater, and the term "increased voltage" means that 110% of the standard supply voltage is supplied to the heater.
Since the voltage is used by the heater to drive the blower, supply voltage variations result in changes in the pressure of the fresh air and the combustion air supplied by the blower. In this sense, there would be a "predetermined" range of pressure of such air corresponding to the predetermined range of the supply voltage.
Finally, the ANSI Standard requires the controls of gas-fired construction heaters to shut the burner off before the CO Standard or an upper temperature limit is violated. For example, the burner must shut off at the appropriate point as more and more area of the fresh air inlet and/or the heated air outlet is blocked. In testing such heaters, such blocking is done by successively placing pieces or strips of 1.5 inch wide adhesive tape across each inlet and/or outlet. This is referred to as the "strip method." Alternatively, one may use an iris-type orifice which symmetrically restricts air flow through such inlet or outlet.
The problem Applicants faced, and which the present invention solves, is the provision, in a gas-fired construction heater which keeps the heated fresh air separate from the products of combustion ("furnace-type"), of structure which enables the heater to meet the ANSI Standard, and especially the CO Standard. Prior patents and so-called salamander heaters (which mix the heated fresh air and the products of combustion--and which meet the CO Standard) of which Applicants are aware have not appreciated the problem of providing a furnace-type gas-fired construction heater which meets the ANSI Standard.
An example is Pelsue U.S. Pat. No. 4,108,143. Although this patent recognizes the goal (in a skid-mounted cable maintenance heater) of efficient use of BTUs, it does discussed, and no tertiary air is supplied to complete the combustion.
Morris U.S. Pat. No. 3,765,398 relates variations of external wind conditions to variations in air flow to a burner. Morris provides an air-flow responsive air valve that shuts an air inlet to the burner's air blower when a gust of wind is sensed.
Ito, et al U.S. Pat. No. 3,757,767 appreciates a "lifting" problem, where the flame lifts off the burner, but solves it by selecting the size of flame holes 35. Secondary air is supplied parallel to the flame from a chamber.
Weiss U.S. Pat. No. 3,747,586 shows a solution to a different problem, and the solution (a variable port to a pilot) is contrary to the ANSI Standard. Weiss also shows a variable shutter to control air to the burner, contrary to the ANSI Standard.
Mayo U.S. Pat. No. 2,561,934 uses an adjustable primary air damper and directs forced air via a tube onto a flame. This shows a lack of appreciation of the ANSI Standards problem.
Velie U.S. Pat. No. 4,848,313 has a goal of improving efficiency of combustion and getting mor BTU output/cubic inch of heater. Velie shows primary and secondary air inlets below a flame spreader that is above a burner.
Velie U.S. Pat. No. 4,651,711 has a main concern of providing a removable burner. Velie uses a flame spreader without air holes at the flame tips. The air inlet is only at one side of the flames and there is no heat exchanger.
Yagisawa U.S. Pat. No. 4,427,367 is directed to avoiding carbon build-up on a fuel nozzle. Air is fed in an annular flow around a plate and along a nozzle and through the plate via openings to prevent carbon build-up.
Jalics U.S. Pat. No. 4,221,557 has a primary goal of sensing improper combustion conditions at a main burner, and turning the main burner off before its flame extinguishes. Jalics uses a secondary burner which extinguishes just before the main flame starts to operate in an incomplete combustion mode. Jalics does not refer to the ANSI Standard, and does not appreciate the requirements thereof as shown by the use of a variable air scoop.